Motion-compensated inter-frame coding has been widely used in various international video coding standards, such as MPEG-1/2/4, H.264/AVC and the emerging HEVC (High Efficiency Video Coding) standard. In an inter-frame coding system, the motion information is derived using one or more reference frames, where the reference frame may be a frame before or after the current frame in the display order. The reference frame used for motion estimation/compensation is always a previously reconstructed frame so that the encoder and the decoder can perform motion compensation accordingly with little or no side information. At the encoder side, frame buffer is used to store one or more reference frames in order to perform motion compensation. At the decoder side, frame buffer is used to store one or more reference frames in order to perform motion compensation as well as to provide the decoded frames in display order. Usually, a DRAM (Dynamic Random Access Memory) is used to implement the frame buffer in a video processing device, such as a video encoder or a video decoder, regardless of whether the video processing device is implemented in a manner of hardware-based, software-based, or hybrid-based.
In older video standards such as MPEG-1/2, one or two reference frames are often used. However, in more advanced coding systems such as AVC/H.264 or the emerging High Efficiency Video Coding (HEVC) system, more reference frames may be used. The size of the frame buffer increases along with the number of reference frames. Furthermore, the size of the frame buffer will also grow with the picture size. For High Definition Television (HDTV) signals, the picture resolution is about six times of the resolution for a standard definition TV signal. Therefore, it is desirable to reduce the storage requirement associated with reference frames. Accordingly, various frame compression techniques have been reported in the literature. The frame compression techniques reported in the literature always use fixed compression ratio for each reference frame. Often, lossy compression is used for frame compression in order to achieve higher degree of data reduction. The noise introduced by lossy frame compression will degrade quality of the reconstructed video. Depending on the nature context of the reference frame, the noise in a reference frame may have different impact on the video quality. For example, the noise in an I-frame may cause more severe quality degradation than a P-frame or a B-frame. Nevertheless, conventional frame compression does not take into consideration of the nature of reference frames. Accordingly, it is desirable to develop an adaptive frame compression technique that can adaptively adjust coding parameters associated with frame compression according to the nature of the reference frame. Furthermore, other information about the underlying video as indicated in the syntax of the bitstream and decoder system information may also be useful for frame compression control.